During the present period of study, the applicants have standardized a powerful culture system for studying oligodendrocyte (OL) differentiation and myelin formation that reflects developmental events in vivo. Using this approach, OL progenitors can be experimentally regulated with regard to proliferation and differentiation, resulting in the routine production of expanded, developmentally synchronized, lineage stage-specific populations of cells. The applicants are using this system to analyze key events that occur as OL progenitors enter terminal differentiation and begin to synthesize myelin membrane. They will now orient their studies towards an analysis of molecular mechanisms of OL terminal differentiation and myelin biogenesis. The specific aims of this next project period are therefore to: 1) identify key components and molecular mechanisms of the OL vesicular trafficking system that are critical for myelin biogenesis, maintenance, and remyelination. In particular, they will study the small GTP-binding proteins that act as regulators of vesicle formation and vectorial transport, identifying novel members of this family, and defining their function; 2) Delineate the mechanism by which fibroblast growth factor (FGF) regulates OL proliferation and differentiation. Emphasis will be placed on identifying the changing repertoire of FGF receptors and co-receptors, and understanding the functional significance of this complexity; 3) Establish an OL-neuronal co-differentiation. Working under the reasonable hypothesis that primary OL differentiation and myelin formation is a model for events occurring during remyelination in pathological situations such as multiple sclerosis, these studies have the strong potential to contribute to clinical intervention to encourage the repair of demyelinated lesions.